In general, polyesters such as polyethylene terephthalates (PET) possess good mechanical, thermal, chemical and electrical properties. Therefore, they have been widely used in preparing magnetic tapes, photographic films, capacitors, electrical insulating materials, wrapping materials, industrial mold products and others.
However, polyesters usually do not have sufficiently high heat resistance for certain electrical application, and they also contain about 2.0 to 2.5 wt % of trouble-causing low molecular weight materials, e.g., a conventional PET resin contains a cyclic trimer and oligomers composed of 3 and 4 to 10 ethylene terephthalate units, respectively, dialkyl terephthalate and unreacted starting materials.
For example, when such polyesters are used as insulating materials in an air-tight chamber such as a freezer, the cyclic trimer and oligomers present in the polyester film may be leached out into the refrigerant, thereby blocking nozzles of the freezer motor and contaminating the refrigerant. Further, when such polyesters are used in magnetic media, the cyclic trimer and oligomers may accumulate on the surface as white powdery deposits or protrusions during the high-temperature processes of calendering and applying a magnetic layer, thereby lowering the performance quality of the final film product, e.g., poor appearance, runnability, wearability, etc. Furthermore, in an injection molding process, the trimer and oligomers may contaminate the mold.
Accordingly, it is desirable to prepare a polyester resin with a reduced content of low molecular weight materials, particularly the cyclic trimer and oligomers, preferably to below 1.0 wt % by weight. However, it is very difficult to reduce their content to such a low level because they are always present in a fixed equilibrium amounts relating to the polyester portion.
Numerous attempts have been made to remove such low molecular weight materials and methods based on extracting the low molecular weight materials using various solvents have been used most frequently.
Japanese Patent Publication No. 23348/1968 discloses a method of removing low molecular weight materials from a polyester film by dipping the film into a heated dimethylformamide solution to extract the low molecular weight materials out from the film, and similarly, Japanese Patent Publication No. 2120/1969 describes a method which comprises dipping a linear polyester film in a heated solution of benzyl alcohol until the content of the low molecular weight materials in the film becomes about 1% by weight or less, and then removing the benzyl alcohol solution containing the low molecular weight materials.
Such extraction methods may be effective in reducing the content of low molecular weight materials when an appropriate solvent is employed. However, they have several disadvantages, e.g., additional solvent-handling steps are required, leading to an increased production cost. Moreover, the properties of the polyester film may deteriorate by the action of the solvent used.
Further, Japanese Patent Publication No. 41327/1984 teaches a method of preventing the generation of such oligomers by adding an end-group-blocking agent during the polymerization step. However, this method has a difficulty in controlling the intrinsic viscosity of the polymer, which may cause decreased productivity as well as poor heat resistance of the finished product.
Japanese Patent Laid-open Publication No. 296860/1990 discloses a method of enhancing the heat resistance of a polyester film by controlling the amount of inorganic particles contained in the resin and the speed of crystallization during the film preparation process. This method has a disadvantage in that an excess amount of stabilizer is required in the polymerizing reaction, causing a longer polymerization cycle time, and hence, low productivity.
Japanese Patent Laid-open Publication No. 263937/1992 describes a method of coating the surface of a polyester film with a layer composed of a mixture of polyester and polyacrylic resins. The polyester film prepared by this method is coated on both surfaces thereof, thereby increasing the production cost, and creating the problem that the coating layer may be abraded after a prolonged use as an insulating film.